Molecular orientation of a material may be dependent on process parameters such as time, temperature, draw-ratio, crystalline content, and the like. For example, it is generally known that drawing of a polymeric material leads to an increase in the molecular orientation of the material and hence the anisotropic mechanical properties of the material, such as its tensile strength, elongation, and rupture energy. In view of this, it is useful to identify the degree of molecular orientation of a material in order to study the effects of these process parameters on the type and degree of orientation, allowing scientists to better understand the molecular behavior and mechanical properties of the material. Further, obtaining the orientation information during processing in real-time, without removing the material from the production process, i.e., on-line, allows for instantaneous process control and optimization. Some benefits of instantaneous process monitoring include a reduced time to develop an optimized process, reduced material waste, and reduced variability in final product, all leading to a more efficient process. Real-time orientation information and on-line measurement are especially important for materials that are dimensionally and morphologically unstable (i.e., due to relaxation, crystallization, etc.), where the properties of the material may change due to changes in time, temperature, etc, for example, where changes in properties can occur in the short period of time it takes to remove the material from the manufacturing process to the location for evaluation of molecular orientation.
Birefringence, also known as double refraction, is a useful technique for measuring molecular orientation of a material, and thus is useful for studying structure-property relationships of various polymeric systems. For example, the effect of various process parameters can be evaluated directly by measuring birefringence. Over the past years, several techniques have been developed to measure this optical parameter. Such techniques include the use of isorefractive immersion fluids, a depolarized microscope with a compensator, a photographic interference fringe method, or even a depolarized monochromatic light source.
One well known technique for measuring the birefringence of a material is optical microscopy, which utilizes visible light having wavelengths in the range from about 400 to about 800 nm, to evaluate the molecular orientation of a material. This technique, which may be used to evaluate materials of various geometries such as planar films or cylindrical fibers, is described, for instance, by Yang et al, in J. Polym. Sci. Polym. Phys. Ed. 20, 981-987 (1982), where the birefringence of stationary fiber samples is measured using Babinet or Berek compensators. Additionally, GB 2,066,458 describes an optical microscopy system using a compensator for on-line measurement of birefringence of fibers in motion. One drawback with either compensator-based method is that measurements are not readily made on fibers in motion. Although GB 2,066,458 discloses an optical microscopy system for on-line measurement of birefringence, it is expected that visual observation of a moving fiber through a microscope is onerous because the fiber would be continuously moving in and out of focus, and the dark fringe that indicates the correct angle on the compensator would be difficult to assess for the moving fiber. In addition, compensator techniques fail on samples with very high or very low birefringence.
Several other techniques using interference-based optical microscopy for measurement of birefringence of fiber samples are described by Roche et al, in Fiber Prod. 12(1), 51-56 (1984) and Textile Res. J. 57, 371-378 (1987); Hamza et al, in Poly. Comm. 30, 186-189 (1989); and Yang et al, in Polym. Sci. Polym. Phys. Ed. 20, 981-987 (1982); and in GB 2,052,049. These techniques are useful for deconvoluting orientation across a fiber, but like the compensator-based methods, interference-based methods are unsuitable for on-line measurement of fibers since it is difficult to observe the interference fringes. For example, observing and counting the fringes may be difficult for a fiber in motion since the moving sample would blur the fringes and prevent fringe-counting.
There are additional drawbacks to using optical microscopy to measure the birefringence of a material. Another drawback is that use of optical microscopy to evaluate the molecular orientation of a sample is unsuitable for dyed materials, since the dye in a sample would absorb much of the spectra of light in the visible light range of wavelengths utilized in optical microscopy. Yet another drawback of using optical microscopy is that the birefringence of very low and very highly oriented materials cannot be evaluated, since there are few reference standards at very low or very high orders of retardation.
Other techniques for measuring the birefringence of a material using a visible light source have been disclosed, for example by Yang, Chouinard and Lingg, in Polym. Sci. Polym. Phys. Ed. 20, 981-987 (1982), who developed a method to measure birefringence of highly oriented fibers using a visible light source Beckman spectrophotometer. Hongladarom and Burghardt, Macromolecules, 26, 785 (1993), and Beekmans and de Boer, Macromolecules, 29, 8726 (1996), report a spectrographic birefringence technique for the determination of orientation of liquid crystalline polymers solutions having high anisotropy. This technique uses a multiwavelength white light source operating in the range of 500-700 nm. In this wavelength range the birefringence is wavelength dependent (known as birefringence dispersion). The birefringence dispersion is also material dependent. The variability of the birefringence dispersion was addressed by these authors several ways. First, the relative birefringence corresponding to a single wavelength (633 nm) is calculated either by fitting the observed spectra with an arbitrary set of equations containing several adjustable parameters (Hongladarom and Burghardt), or by determining the periods of oscillations over a short wavelength interval from which the birefringence is calculated (Beekmans and de Boer). The last method assumes that, for a given material, the birefringence does not change much between two subsequent zero crossings, which is generally not the case for this wavelength range used. Finally, since both approaches rely on normalized intensity measurements, these techniques are very sensitive to the changes in the thickness of the sample which can cause large errors in measuring birefringence. In addition, like the previously described methods using optical microscopy, the visible light spectrographic methods are not suitable for dyed materials.
Additional techniques using monochromatic light sources have been disclosed. Mortimer and Peguy, (Textile Res. J. 64(9), 544-551 (1994), built a device for on-line measurement of fiber birefringence, using a He—Ne, monochromatic laser (632.8 nm) source. U.S. Pat. No. 4,309,110 to Tumerman discloses an apparatus and method for determining optical properties of a substance by passing a beam of linearly polarized monochromatic light through the substance. The polarization vector of light is mechanically caused to rotate at a definite frequency, and the light is measured by a photodetector. The relative phase shift and/or modulation coefficient of this beam after passing through the substance is compared with a reference beam that has not passed through the substance, to effect measurement of linear and circular birefringence. Finally, U.S. Pat. No. 5,319,194 to Yoshizumi et al. discloses a method for measuring birefringence employing a laser that emits two beams at different frequencies. After the beams have passed through the sample, the beams are split by frequency and directed to two analyzers that are polarization sensitive. However, each of the monochromatic light source techniques described above is unsuitable for evaluating the birefringence value of highly oriented fibers where optical retardation can go to very high orders. Also, in laser-based techniques, when measuring small-diameter samples in motion, the signal becomes very erratic due to the moving laser-to-sample contact point.
Despite the teachings described above, there remains a need for a method and/or apparatus to measure the birefringence, both off-line and on-line, of an anisotropic material having very low or very high birefringence, that is not limited by the shape, form or geometric configuration of the material, or by whether the material is dyed or undyed.